In application Ser. No. 07/499,022, an apparatus and technique were disclosed wherein a mass of plastic resin material was injected into a mold cavity which was defined by the mutually opposing faces of a pair of relatively reciprocable members on opposing sides of a plane of the mold, and had a cross section corresponding to the body of the base of the monolith to be molded. In addition, one of the mold cavity defining members had a set of elongated bristle defining branches of the cavity formed therein, which extended from the face of the one mold cavity defining member along longitudinal axes transverse the plane of the mold and terminated at the ends thereof relatively remote from the face of the one mold cavity defining member. When the gas in the cavity was displaced into the branches by the resin material, the gas was discharged in turn to a low pressure zone at the terminal ends of the branches so as to make way for the formation of the bristles of the monolith therein. Moreover, when in certain embodiments of the apparatus and technique, a substrate with a substantially planar face was enclosed in the cavity so that the face thereof was spaced apart from the face of the one mold cavity defining member, the mass of resin material was injected into the space between the faces of the substrate and the one mold cavity defining member to form a plastic monolith adapted as a bristle supporting overlay for the substrate. In fact, in some embodiments, the substrate was secured to the plastic monolith to form a backing therefor, for example, by interposing spacer elements between the substrate and the faces of the mold cavity defining members on the opposing sides of the plane of the mold to substantially surround the substrate with an unoccupied portion of the cavity, into which the mass of plastic resin material was injected to substantially encircle the substrate with resin material in forming the base of the monolith. To form the gripper elements themselves, the outboard end portions of the arms of the brush were enclosed in a pair of mold cavities corresponding to the bodies of the elements, and spacer elements were interposed between the outboard end portions of the arms and the faces of the members to substantially surround the end portions of the arms with unoccupied portions of the cavities. A mass of the resin material was injected into the unoccupied portions of the cavities to substantially encircle the outboard end portions of the arms with the resin material, and when the gas in the cavities was displaced into the branches by the resin material, the gas discharged from the branches at the terminal ends thereof to enable the bristles to form on the bases of the gripper elements. Additionally, to provide an articulated linkage between the gripper elements, the faces of the pair of mold cavities defining members were given a series of serially interconnected cavities having cross sections corresponding to the bodies of the gripper elements and the linkage therebetween, and the mass of resin material was injected into the series of cavities to form a monolith which was referred to as a "cowling," since when the brush was put to use, the monolith was reentrantly folded about the arms to form a taco shell shaped head which could be straddled about a row of teeth and translated along the length of the row while the gripper elements were applied to the outside and inside faces of the teeth to clean them.
The apparatus and technique of application Ser. No. 07/499,022 have proven to be highly satisfactory in making straddle type brushes of the type shown in application Ser. No. 07/499,022, and application Ser. No. 07/924,099, but the molds for the same have also proven to be expensive to manufacture, and therefore, a way has been sought to reduce the expense of the mold, and particularly, the expense of the mold cavity defining member having the bristle forming branches therein.